Effects of transient hypoxia on internal intercostal muscle activity in vagotomized rabbits.

The effects of transient hypoxia on the responses of internal intercostal (IIC) muscle activity before and after surgical denervation of the carotid sinus nerves were studied in the bilaterally vagotomized rabbits. Bilateral vagotomy caused a complete inhibition of IIC activity in 11 of 13 animals. In the vagotomized animals with no expiratory activity in the IIC muscles, lung inflations with low O2 gas mixture (6% O2 in N2) produced an increase in IIC activity, whereas these excitatory effects were remarkably reduced after sectioning the carotid sinus nerves. In addition, the effects of lung inflation with low O2 gas mixture (6% O2 in N2) on carotid chemoreceptor activity were also studied in the vagotomized rabbits. Lung inflations with hypoxic gas caused an increase in carotid chemoreceptor activity. These results indicate that in the absence of proprioceptive input from the lungs, hypoxic stimulation of the carotid chemoreceptors is capable of activating the expiratory activity in the IIC muscles.